1. Field of the Invention
This invention relates, generally, to monitoring bolt tension. More specifically, it relates to a method of evaluating clamping force using surface acoustic waves.
2. Brief Description of the Prior Art
Threaded fasteners are one of the most versatile methods for assembly of structural components, ranging from small toys to large-scale steel structures such as pipelines, bridges and platforms. The standards of the threaded fasteners employ specific terminology for the parameters that define the general geometry of bolts. FIG. 1A shows the most important parameters of a conventional bolt. The pitch is separation between two contiguous threads. The nominal diameter is the largest diameter of the screw thread. The bolt length is measured from the head to the base. The thread length is the distance from the bolt end to the beginning of the screw thread [1].
Even in an assembly of a single structure, a variety of bolts may be actively used, with ranging sizes and material properties. For example, in steel building structures, two primary types of threaded fasteners are used: critical fasteners and non-critical fasteners. For example, in bridges, large high-strength bolts are used to fix base columns, and small lower-strength bolts are used to support access ladders. High-strength fasteners require careful design and comprehensive care for maintenance. The inspection techniques are used to evaluate clamping force, internal stress and even the crack development in the joint. These techniques are defined by strict standards.
Naturally, not all bolts are critical for the operation of the structure. Fasteners loaded with small forces and present in large quantities do not receive the same treatment as the critical bolts, as they have less importance in structural rigidity. Typical maintenance operations, for example tension measurements, internal stress checking and monitoring of crack development, are not practical or regularly practiced due to cost, time constraints, and the vast quantity of the non-critical fasteners [1]. Although failure of a single non-critical, low-strength fastener likely is not a significant threat to the structure's stability, massive malfunction can cause structural problems, such as insufficient stiffness or excessive vibrations, which may lead to structural failure.
A very common cause of structure failure is the self-loosening of the bolted joints [2-5]. It is explained as a “slip” at the thread-plate and head-plate interfaces [1, 3-7, 14]. The main cause of the “slip” is direct and indirect shear loads applied to the threaded joint [6, 7]. In addition to the shear loads, the preload/tension also tends to loosen the bolt due to the generation of loosening moment related to the helical shape of the thread. Another factor that contributes significantly to the loosening process is the elastic deformation of the fasteners and clamped members; member deformation causes “slip” in the fastener heads and increases the loosening moment in the threads [6].
The health or effectiveness of bolted joints can be defined by one primary parameter: the clamping force (CF). The CF is the force that holds the elements of the joint together. If the CF is too low, separation and bolt fatigue may occur. The CF is too low in loose bolts; low CF indicates the acceleration in the natural loosening process [3], meaning loosened bolts tend to loosen even faster than properly tensioned bolts leading to accelerated bolt loosening as the structure ages [3]. Insufficient CF may further lead to joint separation. In the case of separated bolted joints, aggressive fatigue is induced on the remaining bolts due to the increase tensional stress on the remaining bolts [1, 3]. On the other hand, the CF is too high in excessively tightened bolts; excessive CF may produce damages in the underlying structural members, such as excessive distortion or breakage [3].
The CF is generated by the superposition of the individual tensions of the bolts present in the joint. The bolt tension, also referred to as bolt preload, is the actual force that is stretching the bolt body. The preload is related to the relative stiffness of the bolt and clamped members. Maintaining the appropriate tension in bolts ensures a proper CF and maintains good health of the joint, along with safe operation of the underlying infrastructure. CF is proportional to the bolt tension by the combined stiffness of the clamped members and the fasteners. Tension of all the fasteners in a joint builds up the common force that ensures clamping. Hence, bolt tension of all the fasteners present in a joint is critical for CF [1, 3].
There are two different stages in the operation of bolted joints that require tension control: assembly and regular operation. Geometric characteristics like the stiffness or bolt position usually do not change during the assembly or the regular operation. Thus, accurate tension control is necessary to ensure a correct CF [36]. In the assembly process, the tension is controlled to guarantee a correct preload and therefore the correct CF. On the other hand in the regular operation, the tension is monitored to ensure a safe CF level during the joint life.
Several methods have been studied in order to quantify the health of bolted joints. Typically, the torque control method is applied [1, 4-7, 15]. Other methods are turn-of-nut control [4], direct preload control [4], and stretch control [8]. With torque control, manual, pneumatic or hydraulic torque wrenches are used to apply wide range of torque to the bolt [3]. The inherent dependency of this method on various variables such as friction factor, torsion, bending and plastic deformation of the threads reduce the measurement accuracy of the applied tension to as low as 30% [3, 15].
The turn-of-nut control method has two stages. In the first stage, the bolt is tightened with a conventional torque wrench until it reaches approximately 75% of the material ultimate strength [3]. The second stage involves an additional turn of 1800 after the initial tightening. Every 3600 degree that the bolt rotates increases the bolt length (and hence the tension) by the bolt pitch, so the final turn ensures a tension that creates stresses larger than the bolt material yield point [3]. Tension accuracy of 5% is reported with this methodology. However, this method can only be used for bolts made out of ductile materials with long and well-defined elastic deformation regions [3].
Direct preload control method uses direct estimators of tension, such as strain, stress or deformation. Judiciously positioned strain gauges can measure the strain very precisely leading to tension estimation with an accuracy of as high as 1% [3]. However, as noted, these gauges must be precisely positioned at the bolted joints.
Bolt stretch control is another way to quantify bolt tension. This method employs the transit time of ultrasound measurements along the bolt length to quantify the bolt tension. The main advantage of this methodology is that measurement is independent of friction between the fastener and the clamped plate. Avoiding the friction permits it to be almost as accurate as the instrument used to measure the bolt length change [3]. Additionally, this approach makes it possible to monitor the fastener tension levels only by comparing the new length of the bolt to the value recorded during installation [3]. However, as expected, many older structures with no recorded bolt lengths during installation cannot be evaluated with this method. Also, length variations due to irregular surfaces, uneven machinated processes, temperature changes, plastic deformations and bending displacements introduce significant error to the length estimation.
The common issues with all these frequently used methodologies are their limitation to quantify the bolt tension only during the assembly, measurement capability for a single bolt at a time, relatively high cost, and frequent calibration requirement. However, critical and non-critical bolts should be monitored in real-time; thus, one bolt measurement at a time is a significant limitation for large structures with a high number of bolts.
Response analysis of induced vibrations may be used to characterize the general state of bolted joints. For instance, statistical manipulation can be employed to obtain changes in the vibration signals due to bolt preload variation [8, 16]. Recently, signal processing algorithms based on empirical mode decomposition (EMD) have been used to detect changes in vibration signals produced by impact hammers [17]. This method was also validated empirically and by Finite Element Analysis (FEM). Vibration can be used to monitor common problems, such as stability or resistance in a structure, but identifying the location of the problem becomes a difficult task.
Local approaches, such as monitoring the tension of individual fasteners, can be employed as well [18-21]. The deformations on the fastener were measured by using automatic digital image correlation (ADIC) [18]. Also, piezoelectric sensors were installed in fasteners to sense changes in the electromechanical impedance of the bolts [21]. Guided ultrasonic waves were also used to measure bolt tension. Modulation in Lamb waves due to loosened bolts was used to calculate a joint damage index [19]. Transformation between the wave modes due to stress was exploited by [20] with the purpose of calculating stress levels, tension and CF in the threaded joints. These methodologies often require a direct contact with each bolt to be inspected. Furthermore, a majority of the foregoing methods require either disassembly of the components or data collected during the installation in order to establish the tension levels.
The pitch and catch technique refers to the employment of two different piezoelectric transducers to send and receive guided waves. The waves are sent from the transmitting transducer (T) and acquired by the receiving transducer (R) with information about the material present between them. The waves used in the pitch and catch technique are generally guided waves that can be strongly influenced by small variations in the stiffness or thickness of the material [37]. The pristine condition of the part to be evaluated is taken as baseline for variation of the waves. Modifications in amplitude, dispersion, phase or time of flight are indicatives of changes in the structure of the monitored part. Lamb waves of different modes are used commonly with this technique. Limited space between two transducers is a drawback of this methodology. The waves only provide information from the material in between the transducers. Filtering the acoustic signal may be a necessity for very detailed analysis. For instance, [38] develops a filtering algorithm for diminishing ringing effects, which are common in the imaging generation of concrete structures using pitch and catch techniques.
The principle operation of the pulse-echo method is based in the reflection of acoustic waves. The waves generated in the material are partially reflected by holes, corrosion, dibonding and other defects. The reflected waves carry information that is received by the transducer. The time that it takes for the waves to hit and return to the transducer is called time of flight (TOF). The TOF provides the position of the reflective boundary [33]. The amplitude and frequency of the reflected waves may be used to estimate the size and shape of the defects. Pressure waves are normally employed for through-the-thickness pulse-echo scanning and guided waves, such as Lamb waves or surface acoustic waves, are used for longitudinal monitoring [33]. The principal limitation associated with this method is the difficulty of differentiating boundaries that are close to each other. Proximate defects tend to create reflections that superpose while traveling through the scanned material. Separating the individual effect of each one is critical for a good signal analysis [42]. In some cases this procedure cannot be done correctly, leading to errors in the estimation of size and position of the flaws. Also, the waves employed in the pulse-echo approach generally should be low dispersive in nature, such as bulk waves or surface acoustic waves.
Additionally, the need of previous information about the specific bolt (e.g., its acoustic properties) is a common drawback of the foregoing methodologies. In addition, existing methods cannot measure distantly, disabling simultaneous measurement and prolonging the amount of time and cost needed to evaluate tension in non-critical fasteners, to the point that the non-critical fasteners may not be proactively maintained prior to significant damage caused to the underlying structure.
The prediction of a component's operative life is crucial for its mechanical design. Uncertain loads, ambient conditions, material properties or even misuse are some of the cases that a designer has to overcome in order to predict the life of a specific component. Usually, security factors and redundant designs assure structural integrity even in worst case scenarios. These contingencies generate problems such as increased cost, less efficient designs, or over dimensioned structures. Furthermore, designs that support human lives, like airplanes or civil structures, have additional constrains.
The necessity to predict the operative life of components urged the creation of methods that permit the monitoring of the “health” of structures. The methods that are able to do it without damaging the monitored parts are called non-destructive evaluation (NDE). The principal problem associated with NDE is the necessity of off-line evaluation of components. NDE techniques need very controlled conditions during evaluation processes which normally involve disassembly or service leaving of the monitored component. This kind of monitoring is very common in maintenance programs of any kind of machinery or structure [39].
Structural health monitoring (SHM) overcomes this concrete problem: the evaluation of health is done while the component or structure is in operation. There are two types of SHM according to its monitoring approach. The first kind is called passive SHM, which compares the behavior of the structure while is aging with the original or “brand new” behavior [39]. The second uses inspection of the components to find actual problems in the structures and is called active SHM [39].
Passive SHM involves monitoring the general state of the components by measuring vibration or monitoring stress levels in critical locations. Static structures can also be monitored [40]. Active SHM involves integrating the NDE into the structure itself, permitting the evaluation and analysis of the structural health whenever needed and without interrupting the normal operation of the component [37-39, 41-49]. Examples include ultrasonic technologies, piezoelectric wafer transducers, etc. However, the known methodologies utilizing passive and active SHM, including those already described, suffer from the same or similar drawbacks as denoted previously.
Accordingly, what is needed is a method of effectively evaluating clamping force, thereby improving the maintenance capability of critical and non-critical fasteners, by utilizing an ultrasonic reflection method that does not require previous information about bolted joint, thus permitting simultaneous in situ quantification of many bolted joints on the same surface. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.